Abstracts

Rationale: The degree of mossy fiber sprouting after experimental head injury is regionally localized and qualitatively less than the robust, bilateral sprouting observed in models of pharmacologically induced status epilepticus. Less extensive axon reorganization is a more typical representation of the clinical setting but may have functional implications. Epileptiform activity indicative of recurrent excitatory connections between granule cells in the dentate gyrus after trauma has not been adequately described. Methods: Severe controlled cortical impact injury (1.0mm impact depth) was administered to 6-7 wk old mice. Mice were monitored for behavioral seizures from the time of injury until 10 weeks post-injury. After 8-12 weeks, coronal slices were prepared for patch-clamp recordings from granule cells. Spontaneous EPSCs were recorded at -70mV in normal ACSF and in Mg2+-free ACSF containing 100µM picrotoxin. EPSPs were recorded at resting membrane potential. Photolysis of caged glutamate was used to examine local connections between granule cells. Slices were fixed and processed histologically after recordings. Results: Spontaneous behavioral seizures were observed in 43% of mice (n= 13 of 30), and 95% of mice had mossy fiber sprouting at the injury site (n=20 of 21). Spontaneous EPSC frequency was increased in granule cells of slices ipsilateral to the injury that had mossy fiber sprouting compared to slices from control and injured animals without mossy fiber sprouting (control: 0.64 ± 0.1Hz, contralateral: 0.66 ± 0.1Hz, ipsilateral without MFS: 0.65 ± 0.1Hz, ipsilateral with MFS: 2.2 ± 0.2Hz; F(3,64)= 27.1, p<0.001). Epileptiform bursts of EPSCs were observed in 67% of cells in slices with posttraumatic mossy fiber sprouting (n=34 of 51) and 65% of cells also had spontaneous bursts of action potentials (n=17 of 26). Photostimulation in the granule cell layer evoked an increase in EPSCs indicative of recurrent excitation in 5 of 8 cells ipsilateral to the injury (63%). In the same locations where glutamate uncaging evoked EPSCs, action potentials were also evoked in current clamp recordings. Spontaneous bursts of EPSCs were not observed in slices from control (n=5) or injured animals without mossy fiber sprouting (n=14, contralateral; n=23, ipsilateral without mossy fiber sprouting), and photolysis-evoked responses were absent in these cells (n=5). Conclusions: These results suggest that granule cells receive increased recurrent excitatory connections from nearby granule cells within weeks after cortical contusion head injury. The presence of synaptic network activity was associated with mossy fiber sprouting after injury and did not occur in regions where sprouting was absent. Localized mossy fiber sprouting is associated with seizures and local recurrent excitation in the dentate gyrus after traumatic brain injury.